HIGH-SPEED STEAM-ENGINES

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pressure cylinders, and this involves the use of a junk ring as the restraining
member is, of course, not split. There are many varieties of piston rings
manufactured by private firms or specialists, and these are sometimes speci-
fied by users because of the special advantages that are claimed for them.
One of the best known for high-speed engine work is the " Rowan ", of
which details are given.
When junk rings are used, special precautions must be taken to prevent
the possibility of the set screws slacking back and coming adrift. In all
cases tapped holes should be provided in the piston boss, into which eye
bolts can be screwed for the purpose of withdrawing the piston from the
cylinder.
Piston-rod and Crosshead, Slippers and Guides.—The usual
method of fixing the piston to the rod is by means of a nut screwed on to
the end of the latter. The nut is of the castle type, grooves being milled
across the face to take a split pin. The part of the rod inside the piston
boss consists of a parallel part having the same diameter as the top of the
thread, and a taper part. It is bad practice to make the taper too small.
The rod is then difficult to extract from the piston, and in the event of
water getting into the cylinder the rod may burst the piston by wedging
action. The taper is usually made i in 4 on the diameter, and this is satis-
factory for all types of engine. The tensile stress allowed upon the section
at the bottom of the thread is 5000 Ib. per square inch for small engines,
and 7000 Ib. for large engines, the load being taken at full boiler pressure
on the high-pressure piston. Some makers use a high-tensile steel for
piston-rods, and the stress may then be somewhat greater. It should not
be forgotten that such parts as piston-rods and crosshead and connecting-
rod bolts are nicked by the screwed part and have an inherent tendency to
failure there. The stress on the body of the rod is usually very low, as
the diameter is much greater than the screwed part, owing to the taper of
the part in the piston and the shoulder left upon the rod. In addition,
there is an allowance of £ to J in. for re-turning. High-speed engine
piston-rods are short compared with their diameter, and this condition,
combined with the low stress, makes failure by crippling unlikely.
The combination of crosshead, guide, and connecting-rod top end have
given rise to many variations in design with all kinds of screw, wedge, and
cotter adjustments and fastenings, but generally only two types are now
used, both being taken almost unchanged from marine engine practice.
This may be accounted for by the fact that the high-speed engine was to
a great extent developed by firms who had had previous experience in naval
and marine work, where the types referred to have been found quite reliable
and satisfactory.
In one type the crosshead pin is fixed in the jaws of the connecting-rod
and partakes of its angular movements, the bearing remaining fixed and
forming part.of the crosshead. In the other type the bearings are carried
by the jaws of the connecting-rod, the gudgeons being solid with the cross-
head body. Both types necessitate a forked connecting-rod end.